I have the 6600 ... and would sure be sitting up taking notice if it were only a $400 difference to upgrade.
From what I understand there are a few extra perks with the 7700.
The biggest one being the 2 extra inches in the throat.
While your upgrade would be a minimal cost, which would definitely be speaking to me ... I'm always hesitant on buying used, and the problems which may come with it. Will your dealer offer you a month's warranty, so you can give it a good run and know that you have a good machine?
I would definitely look at the 8900, though the cost difference would be higher.
My understanding is that it is similar to the 7700, but much better.
And I'm hoping some others will jump in here with direct knowledge on all three.